It is often desirable to incorporate safety controls in powered closures such as building door operators such that in the event the door encounters an obstacle during its opening or closing movement, the powered operation is either discontinued or the direction of drive to the door is reversed, so as to avoid a jam condition which may cause injury to persons or damage to the door or other equipment. Such devices have heretofore been provided which mechanically sense a jam condition and are commonly employed in applications as garage door operators.
Such arrangements are satisfactory for relatively lightweight door structures as are normally incorporated as garage doors. However, such powered operation is sometimes utilized in connection with relatively massive door closures. One such application is for sealed doors for testing enclosures in which a relatively heavy door is cantileved to be moved overhead between an open and closed position. In the closed position, the door is sealed with respect to the closure structure as by an air pressure sealing arrangement. The sealing of the door precludes the rolling track motion of typical garage doors and requires a simple hinging about an overhead horizontal hinge axis, which requires considerably more power to operate due to the cantilevering of the entire door weight. The relative mass of the door and the overhang mounting of the door reqires considerable operating power, and any dependence on simple mechanical detection of an overload condition in the door drive may not react rapidly enough to avoid the injury or equipment damage sought to be eliminated by the safety circuits.
Furthermore, the above-mentioned mechanical jam detectors require the development of a jam condition in which a reaction force is impressed on the obstacle. Such forces may be moderate for the applications described but would be excessive for many other applications.
Accordingly, it is an object of the present invention to provide a safety circuit arrangement for a power closure operator in which very rapid reaction of the safety reversal is achieved prior to the development of any significant load in the drive system such as to avoid injury or damage even for such door operating systems involving substantial momentum of the door and high operating forces exerted during its opening and closing movement.
It is yet another object of the present invention to provide such a reversing arrangement which is simple and yet highly reliable in operation and which is very sensitive to the development of interruption in motion of the door during opening or closing movement.